'Not one patient has benefited from a cannabis prescription on the NHS.

'The legislation has had no impact on the health of people due to the lack of education of the medical community and overcautious guidelines produced by the Royal College of Physicians and the British Paediatric Neurology Association.'

An open letter from the parents of 39 children with intractable epilepsy – defined as seizures that cannot be controlled – is calling for the BPNA to change its guidance.

The parents have accused the association of ignoring England's chief medical officer, Dame Sally Davies, who recommended medicinal cannabis be available to all eligible epilepsy patients.

This comes after 17 children with the condition were denied prescriptions despite campaigners fighting for their right to the drug in the movement End Our Pain, its director Peter Carroll said.

Alfie Dingley was part of the campaign and is one of just six children Professor Barnes knows to have been given medicinal cannabis since the law change.

One of these youngsters is 12-year-old Billy Caldwell, whose mother had seven bottles of cannabis oil confiscated at Heathrow Airport customs on June 11.

The campaign group United Patients Alliance estimates a million people in the UK take cannabis illegally to help manage their health conditions.

But non-medicinal marijuana can contain a host of contaminants and has an unknown strength.

Jon Liebling – the alliance’s political director – claims by not prescribing medicinal cannabis, doctors are making patients vulnerable to 'the criminal market'.

He added he knows of just two medics who are willing and able to write these prescriptions.

Alfie Dingley was the first person to receive medicinal cannabis in the UK under a special licence before the law change.

Before the legislation came into effect, his mother claims one doctor told her 'if you talk to me about cannabis again I'll report you to social services'.

It then took eight doctors to complete the administrative procedures required to get Alfie the drug, which Ms Deacon compared to 'trying to import plutonium'.

Doctors then asked Ms Deacon to switch Alfie back to CBD products without THC, which she called 'unethical'.

Cannabidiol (CBD) is a component of cannabis that has never been restricted in the UK and is even available on the High Street as an oil or spray.

Up until November 1 the drug Sativex - which contains both CBD and THC - was the only medicinal-cannabis product licensed in the UK and was given to MS patients with spasticity.

Products that contain both CBD and THC may be prepared from cannabis-plant material.

Products include Bedrocan, Bedrobinol, Bediol, Bedica, and Bedrolite, which are all made by the pharma company Bedrocan in the Netherlands.

The 12-year-old sparked a row over the medicinal status of the oil, prompting the Home Office to step in and grant his mother Charlotte an emergency licence for the product that was calming his seizures, which contained THC.

Billy's bottles were confiscated on June 11 after Ms Caldwell brought them in from Toronto.

On the back of the cases of Billy and fellow epileptic boy Alfie Dingley, six, Home Secretary Sajid Javid called for a review into medicinal cannabis.

In a major shift of policy, he announced in July that some products containing the drug would be available on prescription in the UK from the autumn.

On the back of today's change to the law, Ms Caldwell said she wept with joy.

'For me what started off as a journey which was about the needs of my little boy actually turned into something, proved to be something, a lot bigger,' she told Sky News.

'It proved to be the needs of a nation.

'Medicinal cannabis gave me back my right as a mummy to hope, but the most important thing medicinal cannabis has done is given Billy back his right to life.

'Only relatively recently did our Government and country really start to appreciate just how many wee children and people of all ages were affected by the difficulties associated with accessing medicinal cannabis.

'But it became clear it wasn't just about what was perceived to be a small number of very sick children and that medicinal cannabis could make a life-changing or life-saving difference to more than a million people.'

Although thrilled by the law change, Ms Caldwell hopes regulations will be expanded to allow more people to benefit from cannabis-based treatments.

'This is new ground for everybody. We did in a few days what successive UK governments failed to do in more than half a century and made medicinal cannabis legal,' she said.

'Then, as now, politicians didn't realise the complexities involved.

'There's a wide range of conditions, each of which can only be treated by certain forms of medicinal cannabis.'